Breast Cancer remains the most common cancer in North America and the second leading cause of cancer death in women. Radiation therapy (RT) plays an integral part in the treatment of breast cancer with more than half of all breast cancer patients receiving radiation sometime during the course of their treatment. The conventional view of RT has largely focused on the effect of RT on the tumor cells themselves. However, recent studies have demonstrated a critical role for the immune system in determining the response of tumors to RT. Multiple studies have identified macrophages as key cells that regulate the immune response in tumors following RT. Macrophages have multiple different phenotypes that can either support or suppress an ongoing immune response and our preliminary data suggests that targeting a cytokine, interleukin(IL)-4, that controls the development of macrophages that suppress an ongoing immune response enhances the efficacy of RT. The objective of this research proposal is to address the mechanism(s) by which IL-4 blockade enhances the response to RT. The proposal tests the hypothesis that macrophage phenotype regulates RT-induced immune responses and that the efficacy of RT can be enhanced in vivo by IL-4 blockade. To evaluate this hypothesis, the following Aims are proposed: Aim 1: Characterize the effect of IL-4 blockade combined with RT on intra- tumor immune responses in a murine model of breast cancer and Aim 2: Define mechanism(s) of IL-4- regulation of RT responses using a murine model of breast cancer and a 3D co-culture system. We will accomplish these aims using focal RT in a murine model of breast cancer and studying the effects of RT and IL-4 blockade using a combination of flow cytometry, immunohistochemistry, quantitative PCR and ELISA to determine the changes in the immune profile of tumors. We will also elucidate the role of macrophage phenotypes using a 3D co-culture system in which macrophages polarized in vitro or sorted from tumors are cultured with tumor cells to determine how different macrophage phenotypes regulate the tumor cell response to chemotherapy and RT. The significance of this research is that it will provide insights on the tumor immune response to radiation that may lead to new immune-based therapies for the treatment of breast cancer and multiple other solid tumors in which RT plays an integral therapeutic role.